The invention relates to a telescopic shaft, for connecting two joints. In particular, a telescopic shaft comprising an outer part and an inner part which are provided with running grooves of which two are arranged opposite one another and which receive balls for the purpose of axially adjusting the two parts relative to one another and for transmitting torque between the two parts.
U.S. Pat. No. 5,624,318 describes a driveshaft connecting two constant velocity joints into which there is integrated a telescopic shaft or a telescopic shaft portion which compensates for any changes in length occurring when the joint angles are changed. The telescopic shaft comprises an outer part in the form of a sleeve and an inner part in the form of a shaft portion. The inner part adjustably enters a bore of the outer part in the direction of the common longitudinal axis of both parts. In the bore of the outer part there are provided circumferentially distributed outer running grooves which extend parallel to the longitudinal axis. In the outer face of the inner part there are provided inner running grooves which are arranged opposite the outer running grooves. The outer running grooves and inner running grooves are distributed around the longitudinal axis at identical pitches. Each pair of inner running grooves and outer running grooves accommodates a plurality of balls which serve to transmit torque and which together, form an anti-friction bearing for adjusting the outer part and inner part relative to one another in the direction of the longitudinal axis.
For designing a telescopic shaft or telescopic shaft portion, the inner part constitutes the part which determines the pitch of the inner running grooves and the size and number of the balls, taking into account the torque values to be transmitted and the peak loads occurring in a predetermined application, for example in the driveshaft of a motor vehicle. The pitch for the outer running grooves in the bore of the outer part is thus also determined. However, as the outer running grooves are arranged on a larger diameter than the inner running grooves, there occur webs between each two outer running grooves in which, due to the greater circumferential distance of the outer running grooves, there are accumulated larger material masses than between inner running grooves of the inner part which closely adjoin one another in the circumferential direction. For the outer part this means that the disadvantageous distribution results in distortion during hardening. This distortion necessitates subsequent machining. Rough tolerances cannot be admitted because any rotational play or any resistance against displacement changing along the path of adjustment is not acceptable as it adversely affects functioning. Subsequent machining increases costs.